1. Field of the Invention
The present invention relates to the a process for the alkylation of benzene contained in a mixed refinery stream. More particularly the invention relates to a process wherein feeds containing olefins is hydrogenated to remove the olefins and then to alkylation of the benzene with controlled types and quantities of olefins. The process is also defined by pretreating sulfur containing refinery streams by hydrodesulfurization of any organic sulfur contained within the stream. All of the process steps may be carried out in distillation column reactors to take advantage of that mode of operation.
2. Related Information
Ethyl benzene and cumene are currently produced by the reaction of benzene and the respective olefin, ice., ethylene or propylene by acid catalysis. In some known processes the catalyst is highly corrosive and has a relatively short life, e.g. AlCl.sub.3, H.sub.3 PO.sub.4 on clay, BF.sub.3 on alumina, and others require periodic regeneration, e.g. molecular sieves. In addition the exothermicity of the reaction and the tendency to produce polysubstituted benzene require low benzene conversions per pass with large volume recycle in conventional processes.
To overcome many of the disadvantages of the conventional processes a process has been developed wherein the reaction of the olefin with benzene is carried out concurrently with separation of the products by fractional distillation. One embodiment of that process is disclosed in U.S. Pat. No. 5,243,115 which utilizes a reaction system wherein the components of the reaction system are concurrently separable by distillation, using the catalyst structures as the distillation structures. Such systems are described variously in U.S. Pat. Nos. 4,215,011; 4,232,177; 4,242,530; 4,250,052; 4,302,356; and 4,307,254.
In addition, a variety of catalyst structures for this use are described in U.S. Pat. Nos. 4,443,559 and 5,348,710 which are incorporated herein.
The reduction in the lead content of gasolines and the use of lead anti-knock compounds has led to a search for other ways to improve the octane number of blending components for gasoline. The alternatives to uses of lead anti-knock compounds are chemical processing and the use of other additives.
One common process long used by the refinery industry to upgrade raw naphtha to high octane gasoline is catalytic reforming. Because of the multiplicity of the compounds in the raw naphtha, the actual reactions which occur in catalytic reforming are numerous. However, some of the many resulting products are aryl or aromatic compounds, all of which exhibit high octane numbers. The aryl compounds produced depend upon the starting materials which in a refinery are controlled by the boiling range of the naphtha used and the crude oil source. The "reformed" product from a catalytic reforming process is commonly called reformate and is often separated into two fractions by conventional distillations--a light reformate having a boiling range of circa 115-250.degree. F. and a heavy reformate having a boiling range of circa 250-350.degree. F. The aryl compounds in each fraction are thus dependent upon their boiling points. The light reformate contains lower boiling or lighter aryl compounds, e.g., benzene and toluene.
The light reformate is that portion containing benzene and lighter components. Now the complex model for gasoline requires severe reduction of the benzene content of gasoline, while maintaining the octane of the gasoline. One effective means to achieve this is to alkylate the benzene, however the olefin streams for this purpose may be expensive or otherwise employed. Thus in one embodiment of the present invention olefins normally destined for fuel gas are used for the alkylation.
Benzene is also contained in appreciable quantities in such other refinery streams as straight run naphtha and to a lesser extent naphtha from catalytic crackers. The conventional method of producing benzene for the alkylation reaction has been the solvent extraction of benzene from such mixed refinery streams followed by distillation to separate the benzene from higher boiling aromatic compounds such as toluene and xylenes which are also present in the extracted streams. Additionally a considerable amount of energy must be expended to separate the solvent from the extracted aromatics.
The alkylation of benzene contained in a naphtha from a catalytic reforming unit has been suggested in U.S. Pat. No. 5,082,990 which also suggests utilizing the previously described concurrent reaction/distillation. However, the alkylation of the benzene is simply to reduce the benzene concentration to meet expected EPA requirements and improve octane. The olefins used for the alkylation are contained in another mixed refinery stream which generally consists of an off gas from a catalytic cracking unit. The melange of olefins along with the mix of aromatics leads to a complex mixture of products which may include alkylated toluene and dialkylated products. This is not a problem in the disclosed process since the purpose is to produce gasoline.
More recently it has been found that a primary cause of catalyst deactivation in aromatic alkylation processes is the presence of high concentrations of olefin. The present inventors have determined that an exponential relationship exists between olefin concentration and catalyst life. Thus clearly the alkylation requires a careful control of the olefin reactant. Further, the deactivation is more rapid with higher olefins above C.sub.4.
The light reformate itself also contains olefinic compounds which are higher boiling. Also the benzene from steam or catalytic cracking also contains appreciable olefins. The higher boiling olefins are longer chain unsaturates which can also react either with the aromatics or with themselves. Regardless of source, the reaction of these higher olefinic compounds is undesirable because they coke up and foul the catalyst causing accelerated catalyst aging.
A problem associated with the use of straight run naphtha or naphtha from a steam or catalytic cracking process is that the naphtha may contain sulfur contaminants, such as thiophene, which in the benzene boiling range in cracked naphthas or mercaptans in straight run naphtha. Thiophene is an unwanted contaminant in either ethyl benzene or cumene. Sulfur contaminants, such as may be found in a straight run naphtha directly from a crude distillation column, may also be mercaptans which are poisons to olefin hydrogenation catalysts.
It is an advantage of the present invention that benzene in a straight run naphtha or reformate stream is alkylated to ethyl benzene or cumene without the extra solvent extraction step.
It is another advantage of the present invention that the olefins in the reformate or straight naphtha stream are hydrogenated to increase catalyst life.
It is another advantage of the present invention that organic sulfur is removed from the naphtha fraction prior to hydrogenation to prevent poisoning of the catalyst.